Method and means for attenuating shock waves propagating within a solid

ABSTRACT

A method is disclosed for attenuating shock waves propagating within solids which waves are defined by an ambulatory threedimensional zone of high stress comprising a plurality of parallel zonal strata oriented normal to the direction of wave propagation, each stratum comprising a plurality of interspersed sets of zonal regions. The method of attenuation comprises the acts of sequentially scattering the sets of zonal regions in each of the zonal strata. Means are also disclosed for attenuating shock waves of predeterminable maximum pulse length propagating within a solid medium of predetermined shock wave speed. The attenuating means comprises a solid structure of shock wave speed at variance from that of the solid medium. The solid structure comprises a set of juxtaposed protuberances in abutment with the solid medium which protuberances span the space between two substantially parallel planes spacially separated a distance in excess of the predeterminable shock wave pulse length.

United States Patent (72] Inventor Edward Y. Harper Los Altos, Catil. [2|] Appl. No 824,586 [22] Filed May 14, I969 [45] Patented July l3, I971 {73] Assignee Lockheed Aircraft Corporation Burbank, Cnlfl.

[ 54] METHOD AND MEANS FOR ATTENUATING SHOCK WAVES PROPAGATING WITHIN A SOLID 5 Claims, ll Drawing Figs.

[52] U.S.CI. l09/l, 109/80, lot/404, 1811.5 [51 I Int. Cl E04b 2/04 [50] FieldotSearch....... l09/l,80, 85,495; 89/36, 36 A; l8l/.S; l6l/404; 174/35 [56] Relerences Cited UNITED STATES PATENTS 2,405,590 8/1946 Mason 109/1 X $395,067 7/1968 Lane 89/36 X 'PRESSURE SPlLLlNG" 5 Primary Examiner-J. Karl Bell Attorneys-Robert 8. Kennedy and George C Sullivan ABSTRACT: A method is disclosed for attenuating shock waves propagating within solids which waves are defined by an ambulatory three-dimensional Zone of high stress comprising a plurality of parallel zonal strata oriented normal to the direction of wave propagation. each stratum comprising a plurality of interspersed sets of zonal regions. The method of attenuation comprises the acts of sequentially scattering the sets of zonal regions in each of the zonal strata.

Means are also disclosed for attenuating shock waves of predeterminable maximum pulse length propagating within a solid medium of predetermined shock wave speed. The attenuating means comprises a solid structure of shock wave speed at variance from that of the solid medium. The solid structure comprises a set ofjuxtaposed protuberances in abutment with the solid medium which protuberances span the space between two substantially parallel planes spacially separated a distance in excess of the predetenninable shock wave pulse length.

" PRESSURE SPlLLING PATENIEflJuLlalsn 3 592 147 SHEET 1 OF 5 LOW SPEED men SPEED UNIFORM seems SEGMENT SPEED DIFFUSED SEGMENTS 'j" LATERAL"SPILLING" OF STRESS INCIDENT SCATTERED FRONT FRONT FIG 1B FIG 1A FRESiSURE SPILLING" PRESSURE SPILLING" INVEN'I'ON. EDWARD Y. HARPER Attorney PATENTEnJuuslsn 3.592.147

sum 2 OF 5 (LOWER SPEED) Fl G- 3 ALUMINUM TIME 4 LEAD ALUMINUM PRESSURE m kbar ASA FUNCTION OF TIME IN put. H

0 ME l lbl/ltH/UP L 7 LEAD EDWARD Y. HARPER ATENTEMuuam: 3.592.147

sum 3 or 5 ALUMINUM pox PRESSURE "KM! F-l an 2.3 i l mil I mom? H 1 1 TIME #586,

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SHEEI 5 [1F 5 SLOWER MEDIUM FASTER MEDIUM Fl G- 9A FASTER MEDIUM 2 SLOWER MEDIUM F l G 9B Attorney METHOD AND MEANS FOR ATTENUATING SHOCK WAVES PROPAGATING WITHIN A SOLID BACKGROUND OF THE INVENTION This invention relates to methods and means for attenuating shock waves propagating within solids. The term shock waves" is meant to include relatively low amplitttde stress waves of sonic speed as well as relatively high amplitude stress waves of supersonic speed.

Shock waves may be introduced into solids by various mechanisms such as explosive loading, mass impact and electromagnetic energy deposition. The subsequent interaction of these waves with the boundaries of the solid and with each other produces a variety of stresses including those of shear, tension and compression. These stresses may result in severe damage to the structural integrity of the solid itself as well as to other structures in the immediate vicinity thereof. For example, a compressive shock wave propagating in a solid will be reflected from a free surface thereof a free surface" demarcating the solid medium from an ambient gaseous medium or a vacuum. The reflected shock, which is a tension wave, may produce such severe stresses as to cause a complete separation of a portion of the solid iii the region adjacent the free surface. This effect, which is generally termed "spall," may be not only destructive to the solid itself but also to other objects in the vicinity of the free surface due to high-velocity separation of solid material adjacent the free surface.

Heretofore, most of the study relating to spall has been in the nature of basic research directed towards an understanding of the dynamics involved. Reported studies include the article titled Influence of Stress History on Time-Dependent Spall in Metals which appeared in the June, 1964 issue of the AIAA Journal, and the article titled Experiments on the Mechanism of Spall" which appeared in the I963 ASTM Material Science Series 5, Special Technical Publication No. 336, Symposium on Dynamic Behavior of Materials. Beyond measuring the stresses necessary to produce spall and the response of various materials to shock loading, most work has been of an academic nature. Though metallurgists have been particularly intrigued with certain microscopic aspects of the material behavior, very little has been done of a practical nature towards achieving ways of preventing or reducing the potential damage which solids may suffer when subjected to shock waves.

Present day efforts towards the alleviation of the dangers posed by the phenomenon of spall have been effectively limited to the employment of well-known civil engineering techniques for providing structural support. This art, while highly developed in dealing with problems of mass impact and steady state conditions of internal stress, offer little if any aid in alleviating the potential damage ofspall. The recent advent of encrgy-absorbcnt materials has provided a possible answer to the problem. These materials typically comprise porous but rigid foams which dissipate mechanical energy from an in cident shock wave when they are crushed. But as they are purposefully designed to be crushable, they are inherently weaker as structural elements than the solid material from which they are formed. Of more importance, however, is the fact that once these materials have been crushed by an incident shock wave, they no longer possess the capacity to dissipate mechanical energy from subsequent shock waves. This vulnerability renders these materials useless in many applicattons.

SOME OBJECTS OF THE INVENTION Accordingly, it is an object of the present invention to provide methods and means for attenuating shock waves propagating in solids.

Another object of the invention is to provide improved methods and means for decreasing the vulnerability of solids to spall.

Yet another object of the invention is to provide methods and means for attenuating a series or succession of shock waves propagating within a solid.

Another object of the invention is to provide means for at tenuating shock waves propagating in solid structures which do not compromise the strength of the structures through the introduction of voids therewithin.

Still another object of the invention is to provide an improved shelter having less vulnerability to spall than shelters of the prior art.

SUMMARY OF THE INVENTION Briefly described, the present invention is one of methods and means for attenuating shock waves propagating within a solid. Where the shock wave is defined by an ambulatory three-dimensional zone of high stress comprising a plurality of parallel zonal strata oriented normal to the direction of wave propagation, each stratum comprising a plurality of interspersed sets of zonal regions, the method of attenuation comprises the acts of sequentially scattering the sets of zonal regions in each ofthe zonal strata.

Shock waves of predeterminable maximum pulse length propagating within a solid medium of predetermined shock wave speed may be attenuated by means comprising a solid structure of shock wave speed at variance from that of the solid medium. The solid structure comprises a set of juxtaposed protuberances in abutment with the solid medium. The protuberances span the space between two substantially parallel planes spacially separated a distance in excess of the maximum predeterminable shock wave pulse length.

BRIEF DESCRIPTION OF THE DRAWING F l (iS tA an d II) are sequential diagrams ofa portion ofa shock wave front being scat tered. m i i "T FIG. 2 is a diagrammatical view of a wave front propagating in two media of dissimilar shock wave speeds.

FIG. 3 is a diagrammatical view of a shock wave being attenuated in accordance with principals of the present invention.

FIG. 4 is a composite illustration which graphically presents measured attenuation obtained through an actual use of principals of the present invention.

FIG. 5 is another composite illustration which graphically presents measured attenuation obtained through an actual use of principals of the present invention.

FIGS. 6 and 7 are prospective views of two preferred embodiments of means for attenuating shock waves in accordance with the present invention.

FIG. 8 is a cross-sectional view of a shelter incorporating means of the present invention.

FIGS. 9A and 9B are two diagrammatical views comparing the effects of shock waves moving from a solid medium of lower shock wave speed to one of higher shock wave speed and vice versa.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When a portion of a shock wave propagating within a solid medium encounters a region of the solid having a higher or lower shock wave speed than the region in which it was propagating, the encountering portion of the wave is scattered, typically being both reflected and refracted. An overall result of the total encounter may be a lateral spreading of kinetic energy. Redistribution in the intensity ot'a wave resulting from such an encounter is referred to as diffraction which term implies that energy has been carried to regions which it could not otherwise reach were the wave propagation strictly rectilinear. It occurred to applicant that this phenomenon might be put to use in attenuating shock waves by sequentially scattering certain portions thereof. This would break the shock wave front into sets of relatively highand low-speed segments as may be visualized with respect to a single scattered portion by reference to FIGS. IA and 18. FIG. 1A shows a shock wave front moving from left to right, all segments of which are traveling at a uniform speed. In FIG. 18 the front is shown to have been broken up into front segments one of which segment is traveling at a higher speed than the other segments. Diffused front segments connect adjacent highand low-speed segments. It is these diffused front segments which have a lateral component of movement as shown by arrows 4 at which lateral spilling" of stress occurs.

The characterization of the diffraction mechanism in terms of "spilling" is intended to convey in physical terms what otherwise could only be stated in mathematical terms. The concept of stress spilling in conjunction wilh wave diffraction is based on the analysis of]. W. Craggs of Kings College, Newcastle upon Tyre, England which was published in I956 in volume 237 of the Proceedings of the Royal Society (London) at page 372. His mathematical analysis, which was based on purely imagincd physical conditions, provides a useful descriptive tool in understanding some ofthe mechanics ofthe present invention.

The basic idea expressed by Craggs may be understood by reference to FIG. 2 in which acoustic waves are shown to emanate from point 0. These waves propagate through two media M and N having an interface along the X axis. Though the wave propagates omnidirectionally, for the sake of clarity we shall only consider that component which propagates in the plane ofthe figure.

Medium M is characterized as having a faster wave speed than that of medium N. The wave front in medium M is represented at one instant in time by the expanding semicircle R'CR. A portion of the wave front in medium N is shown by an expanding circular arc KDK which forms a portion of semicircle ADA. Connecting these portions of the advancing wave front in medium N are diffused wave segments R'K' and RK which are set up by diffraction and which permit spilling of stress from medium M into regions R'A'K'R' and RAKR as shown by arrows 5. As a result of this spilling action the wave front will advance in an oblong fashion bounded by R'C RKDK'R.

Applicant as successfully used the just-described phenomenon of spilling to practical advantage in the attenuation of shock waves propagating within solids. In accordance with principals of the present invention the shock wave is broken up into interspersed sets of relatively highand lowspecd segments. This enables lateral spilling to occur which relieves stress from regions of the advancing shock wave located behind the relatively high speed segments and, in addition, causes portions of the wave front to turn upon itself. By breaking the wave up into alternate highand tow-speed segments with diffused segments connecting each adjacent highand Iow speed segment, the two diffused segments bounding each lowspeed segment are caused to converge upon both themselves and upon the low-speed segment. This condition creates numerous encounters of various segments ofthe shock wave along the advancing shock wave front thereby reducing the magnitude and direction of the material particle s which, previous to the encounter, were moving rectilinearly at high speed. Contemporary understanding of shock wave phenomenon leads to the conclusion that when the converging wave front segments collide and reflect from each other, the high-speed particles therebehind within the wave are signaled to reduce speed and thus shock amplitude.

The just described method of shock wave attenuation may be more clearly understood by reference to FIG. 3 which depicts in cross section a representative portion of means ac tually constructed and used by applicant in performing such attenuation. The means shown consists of two solid blocks of aluminum and lead which abut each other along mating wedge-shaped surfaces thereof. As aluminum has a nominal shock wave speed of 5.2 kilometers/second and lead a nominal shock wave speed of only L9 kilometers/second, it was hoped that a shock wave of pulse length 1 moving to the right as indicated by arrow 8 would spill laterally after encounter with the wedge-shaped interface. Wedge depth d was made in excess of pulse length l; wedge angles 9 were approximately 60.

The structure was placed in a 28 -foot gas gun and its aluminum free surface bombarded by a supersonic shock wave of approximately 3/32 -inches pulse length and I0,000 atmospheres pressure. A quartz pressure gage affixed to the lead-free surface recorded a shock wave of l ,200 atmospheres of maximum pressure for an 88 percent attenuation in shock wave amplitude! In addition, a large traverse pressure gradient was actually measured substantiating the aforementioned, anticipated wave mechanics immediately following the encounter of the wave front with the wedge-shaped interface. This startling degree of measured attenuation, coupled with the verification of lateral spilling, spurred applicant onto more refined experimentations. Though this work has not yet revealed many optimum, structural configurations, it has led to the establishment of the following parameters for means uniquely suited for affecting attenuation:

l. The depth of the wedges should exceed the maximum shock wave pulse length to which the structure might be subjected. This insures strong diffraction of the entire wave incident upon the wedges rather than just the region adjacent the wave front. Furthermore, a relatively large wedge depth provides an extended period of time during which the incident wave is being diffracted.

2. The disparity in wave speed of the abutting solids,should be relatively large because this will insure a relatively large spacing between adjacent high and low wave speed segments. This increases the dimension of the diffused wave segments permitting large lateral spilling.

. A succession of wedges along the path of wave propagation is desired. The attenuation effect by a series of such wave encounters is cumulative.

The relative positions of the higher or the lower shock wave speed material along the path of wave propagation has not appeared to be of significance. With either relative positioning the wave will turn upon itself within the lower shock wave speed wedges as will hereinafter be further explained.

With reference again to FIG. 3 the mechanics by which attenuation is accomplished may be visualized. Wave front ll of shock wave [0 is first seen at the left of the figure to be advancing rectilinearly within a solid aluminum medium towards the wedgeshaped interface with the solid lead medium. Spaced portions of the wave front will first encounter the lead at points [3 and be scattered, a portion of the wave being reflected and a portion being refracted. As the wave continues to advance additional portions of the front will be scattered upon encounter with the interface as well as portions of the wave itself behind the scattered portions of the wave front. As previously explained in FIGS. l and 2 lateral spilling will occur creating diffused wave front segments.

At the right of FIG. 3 shock wave 10 is shown in encounter with the wedge-shaped interface between the aluminum and lead bodies with portions of both wave front 1 I and rear [2' ofthe pulse in both media. That portion of the wave which has not yet encountered the lead is still propagating rectilinearly while that portion which has been refracted when scattered by encounter with the lower shock wave speed medium, and is now propagating within the lead, is seen to have a lateral component of advance. Within each lead wedge two portions of wave from H are seen to be converging upon one another. As previously explained, it is believed that this serves to relieve stress immediately behind the relatively high speed segments of from H which is still propagating within aluminum wedges. It is also believed the opposition of the convergent frontal segments within the lower speed wedges adds to the reduction in net amplitude of the wave.

FIG. 4 shows the recorded attenuation when two composite solids I6 and 17 of aluminum and lead were subjected to a shock pulse of l0,000 atmospheres. The interface between the two materials of solid [6 was planar whereas the interface of the two materials in solid [7 was corrugated. In each case the aluminum was l-inch thick, as measured in the direction of wave propagation, and the lead was one-eighth inch thick. The depth of the corrugation in solid [7 was flve-sixteenths inch; the wedge angles were each 60".

Solid [6 was subjected to a shock wave pulse of about I microsecond in breadth propagating at about 5.4 lTtI'tL/p, sec. A pressure-sensitive gage affixed to the free surface of the lead recorded H1000 atmospheres of maximum pressure as shown in the graph to the right of solid 16. Solid l7, incorporating means of the present invention, was subject to the same shock wave pulse. As seen in the graph to the right of this solid, a pressure-sensitive gage affixed to the free surface of the lead here recorded only 1,200 atmospheres of maximum pressure. Measurements showing the presence of traverse pressure gradients along the wave front indicative of spilling are shown in FIG. 5.

Three preferred embodiments of means for attenuating shock waves in accordance with the present invention are shown in FIGS. 6, 7 and 8. The structure shown in H6. 6 com prises solid material M relatively low shock wave speed such as loose earth (0.3 km./scc.), concrete (3 krnjsec.) or lead (1.9 kmJsec.) having a corrugated surface in mating abutment with the corrugated surface of solid material M, having a substantially higher wave speed such as copper (4.0 km./sec.), steel (5 km./sec.), aluminum (5.2 km./sec.), berylium (l0 km.lsec.). The opposite surface of M, is also corrugated and is in mating abutment with the corrugated surface of solid material M, which has a lower shock wave speed than M, such as those materials fomiing M,. It should be understood that it does not matter whether the wave speed of M, is greater or less than that of M, or whether M, has a shock wave speed in excess of or less than M so long as there exists a significant variance in the two materials. This may more clearly be appreciated by reference to FIGS. 9A and 9B in which an incident shock wave 19 propagating to the right is attenuated in pressure amplitude to that amplitude of transmitted shock wave 20 by means of the perturbation created in traversing interfaces 21. in FIG. 9A the shock wave moves from a solid medium of lower to one of higher shock wave speed whereas the reverse is pictured in FIG. 9B. in either case lateral spilling occurs as shown by arrows 22 which contain convergent directional components. it should, of course, be understood that the illustrated contour of the wave front here, as in FIG. 3, is only an approximation, a precise physical determination being beyond present day measurement.

FIG. 7 presents another of the many alternate configurations which may be constructed with economy. Here the abutting surfaces of solid materials M and M of variant shock wave speed form a set of protrubing cones 18 rather than a corrugated interface.

FIG. 8 shows how principals of the invention may be incorporated into a specific structure. Here is shown a shelter comprising a discshaped base 25, cylindrical walls 26 and a discshaped roof or lid 27. Each of these structural elements are formed by two solids of dissimilar shock wave speeds which abut one another along a corrugated interface between interspersed sets of juxtaposed protubcrances. Depth 28 of the protuherances exceeds the anticipated pulse length which in cident shock waves might reasonably be expected to possess. The corrugations in base 25 and lid 27 may be either annular or rectilinear. During World War ll many persons were killed by the phenomenon of spall in German air raid shelters. Thus the shelter, which is embedded in earth, is quite suitable as a retreat for personnel whenever the possibility of bombardment exists.

There are, of course, almost an infinite number of particular contours the interface may take as well as combinations thereof in utilizing the principals of the invention. Thus it should be clearly understood that the preferred embodiments shown in the drawing are merely illustrative of principals of the invention. Quite obviously a host of variations and modifications are operatively feasible without a departure from the spirit and scope of the invention as set forth in the following claims.

What I claim is: V l. A shock-wave-pulseattentuatmg barrier comprising:

two different solid materials overlying one another,

said two different solid materials having significantly different shock wave propagation velocities,

said two different solid materials directly continuously abutting one another at an interface defining a multiplicity of juxtaposed angular protuberance means of said materials extending into one another for a distance in excess of the maximum shock wave pulse length in at least one of said two different materials,

said protuberance means being shaped to break up the shock wave into a multiplicity of adjacent highand lowspeed segments which converge upon each other to attenuate the shock wave.

2. The shock wave pulse attenuating barrier of claim 1 wherein said protuberance means are wedge-shaped in cross section in the direction of propagation of the shock wave pulse.

3. The shock wave pulse attenuating barrier of claim I wherein said protuberance means extend into one another for a distance of between 0.0l and 1.0 meters.

4. The shock wave pulse attenuating barrier of claim I wherein one said material has a shock wave propagation velocity at least twice that of the other said material.

5. The shock-wave-pulse-attenuating barrier of claim 1 wherein one said material comprises a metal of high shock wave propagation velocity selected from the group consisting of aluminum, steel, copper and beryllium, and wherein said other material comprises material oflow shock wave propagation velocity selected from the group consisting of earth, concrete and lead. 

1. A shock-wave-pulse-attentuating barrier comprising: two different solid materials overlying one another, said two different solid materials having significantly different shock wave propagation velocities, said two different solid materials directly continuously abutting one another at an interface defining a multiplicity of juxtaposed angular protuberance means of said materials extending into one another for a distance in excess of the maximum shock wave pulse length in at least one of said two different materials, said protuberance means being shaped to break up the shock wave into a multiplicity of adjacent high- and low-speed segments which converge upon each other to attenuate the shock wave.
 2. The shock wave pulse attenuating barrier of claim 1 wherein said protuberance means are wedge-shaped in cross section in the direction of propagation of the shock wave pulse.
 3. The shock wave pulse attenuating barrier of claim 1 wherein said protuberance means extend into one another for a distance of between 0.01 and 1.0 meters.
 4. The shock wave pulse attenuating barrier of claim 1 wherein one said material has a shock wave propagation velocity at least twice that of the other said material.
 5. The shock-wave-pulse-attenuating barrier of claim 1 wherein one said material comprises a metal of high shock wave propagation velocity selected from the group consisting of aluminum, steel, copper and beryllium, and wherein said other material comprises material of low shock wave propagation velocity selected from the group consisting of earth, concrete and lead. 